1. Field of the Invention
This invention relates to the use of radiant energy for the removal of paint and the like from metal, wood, synthetic or composite substrates and more particularly to an improved method and apparatus for the control of the removal process.
2. Description of the Prior Art
Conventional Techniques for the removal or "stripping" of paint and the like include manual scraping, mechanical grinding, blasting with sand or other abrasives and the use of chemical strippers. All of such techniques are costly in terms of labor or materials or both. In addition, all of such techniques tend to generate a substantial waste product which must be disposed of and which, at least in the case of chemical strippers, may be a toxic waste requiring special disposal procedures.
More recently, the intense radiant energy provided by lasers and arc lamps has been used to remove paint and other coatings from metallic substrates with good results. The CO.sub.2 laser, for example, operates at 10.6 micron wavelength which couples well to painted surfaces. The laser beam may be precisely controlled in terms of both power and position. The power of the laser may be selected to remove a top surface only of the paint by vaporization, thus requiring repeated pulses to remove the full depth of the paint layer. In addition, the position of the laser beam can be selectively programmed to remove paint only from certain areas leaving adjacent areas untouched. The vaporized paint may be easily removed in its gaseous form. To avoid degradation of the substrate due to heat, the laser beam may be pulsed at a low rate or moved through a raster over a large area thus allowing an individual area to cool between intervals when it is subjected to the laser beam. The exact nature of the raster and the exact number of pulses required to remove a layer of paint depends on system parameters such as the power of the laser, the size of the laser beam, the thickness of the paint, and the type of substrate upon which it is deposited. Where the substrate is aluminum, laser paint stripping has been carried out with good results because laser energy at 10.6 microns wavelength does not couple well to aluminum and the aluminum in fact acts as a mirror reflecting the energy away when all of the paint has been removed.
However, all of the above stripping techniques, including lasers, have only limited success on composite substrates such as carbon epoxy materials or other plastic reinforced materials. Such plastic reinforced materials are susceptible to mechanical damage and will be attacked by most chemical strippers. In addition, laser energy at 10.6 micron wavelength couples more efficiently to such materials than it does to the paint itself thus providing the potential for serious damage to the substrate by the laser beam.
Aircraft bodies, for example, now include substantial portions of carbon epoxy materials and are painted for a variety of reasons, including aesthetics, identification, camouflage, and performance. Such paint deteriorates under the action of weather and the mechanical forces to which the aircraft is subjected in use thus requiring periodic refurbishment. Often, the deteriorated paint is simply covered up by a new coat and many layers of paint may be built up before paint stripping is required. However, during the service life of an aircraft, many complete paint strippings are necessary. Prior to any such stripping the aircraft will not have a uniform coating of paint. Instead it may have several layers of paint in one spot and only one layer in others. The paint may be five to ten times thicker in some areas due to decals, insignias or other markings on the aircraft and in other areas it may have a hatch cover, underlying repair, or special materials under the paint. All of the above problems complicate the removal of paint from the aircraft and make it impossible to automate the paint removal process according to the teaching of the prior art.
The objects of this invention include the control and automation of the removal of paint and the like from a substrate without damage to the substrate.